See-through optical sights for simulation, training, and close combat (distances less than ˜1000 m) based on collimated optics have been in use for years. [1—Elementary optics and applications to fire control instruments: May, 1921 By United States. Army. Ordnance Dept, page 84, 2—R. P. Grauslys, A. R. Harding, ‘Weapon Aiming Device’, U.S. Pat. No. 7,530,192 B2 (2009)]. The advantage of collimated optics based sights is in the reticle image appearing in the same plane or close to the target plane. This makes the targeting more precise and comfortable. Usually, a see-through monocular optical sight consists of a reticle illuminated with some small light source, reflective and/or refractive optics to create a magnified virtual image of the reticle for a viewer's eye. The simplest type of reticle is a “red dot”—just an LED or laser diode as a reticle. The disadvantage of regular see-through sight optics is in low light throughput due to the unavoidable losses of semitransparent optics for both see-through and reticle image. Implementing narrowband reflecting dichroic mirrors mitigates this problem, but requires narrow bandwidth laser diodes. Implementing laser diodes in sights is questionable as accumulated exposure to bright laser light/scatter in the visible spectrum range can bring to damage of the shooter's eye. Speckled reticle image is also an issue.
Holographic optics based sights implementing high efficient (>90%) thick holograms have higher light throughput [3—R. Bhatt, ‘Testing of Holographic Optical Elements for Holographic Gun-Sight’, Optics and Lasers in Engineering, Vol. 46, pp. 217-221 (2008), 4—W. R. Houde-Walter, ‘Head up Display for Firearms”, U.S. Pat. No. 7,721,481 B2 (2010)]. The deficiency of existent holographic sights is in implementing laser diodes to avoid dispersion and in rather complicated multi-element optics needed to compensate aberrations [5—A. M. Tai, E. Sieczka, ‘Lightweight Holographic Sight’, U.S. Pat. No. 6,490,060 B1 (2002), Assignee EOTech Inc.]. This sight optics, with the reticle image recorded in the hologram, requires individual time consuming alignment of each element, and applied laser diode creates the coherent reticle image with significant speckle that is negatively affecting the image perception.
An edge-illuminated substrate holographic approach with a single hologram for see-through reticle image creation was proposed [6—J. Upatnieks, ‘Compact Holographic Sight’, Proc. SPIE, Vol. 883, pp. 171-176 (1988)]. Such approach indeed allowed for a see-through imagery with simultaneous view of the outside world, however the image quality was rather poor for a broad-band light source, mostly because of the color dispersion created by a single hologram for a broad-band source; for a laser-based (narrow-band) source, an unwanted shift in virtual image position was observed with a drift in laser diode wavelength due to, e.g., external temperature changes. A single-hologram approach was followed by some developers [7—Simmonds, 8—Takeyama, 9—Kasai] for compact see-through head (helmet)-mounted displays (HMDs) using either a single laser source for object illumination or proposing very complicated hologram recording geometries for generating aspheric recording wave-fronts which are rather complicated and costly to be implemented in practice. While providing substantially much more field-of-view that is actually required, e.g., for a see-through weapon sight, it is understood in the art that these developed designs, with a reduced field-of-view and an extended eye-relief, can be used as see-through sights with an illuminated (or, holographically-recorded) reticle replacing a micro-display required as image source for HMDs.
Another approach uses diffractive elements placed on a transparent waveguide to create an enlarged see-through virtual image for a viewer [10—P. Repetto, E. Borello, S. Bernard, ‘Light Guide for Display Devices of the Head-Mounted or Head-up Type’, U.S. Pat. No. 6,825,987 B2 (2004), 11—T. Levola, Method and Optical System for Coupling Light Into a Waveguide, U.S. Pat. No. 7,181,108 B2 (2007)]. While providing a see-through capability, such approach suffers from stray light generated by unwanted diffraction into undesired diffractive orders that substantially decreases the image quality and contrast.
Another approach uses partially reflective elements placed at some angle on a transparent waveguide [12—Ya. Amitai, ‘Substrate-Guided Optical Beam Expander’, U.S. Pat. No. 6,829,095 B2 (2004)]. While creating a see-through imagery, fabrication of such elements in mass quantities can be prohibitively expensive, and providing a needed long eye-relief (˜100 mm) is not possible.
In an attempt to remove the color dispersion/shift of the imagery, another approach was introduced that uses two substrate-guided coupled holograms for a see-through image creation [13—Ya. Amitai, A. Friesem, I. Shariv, ‘Planar Holographic Optical Device for Beam Expansion and Display’, U.S. Pat. No. 6,169,613 B1 (2001), 14—F. Dimov et al., ‘Holographic Substrate-Guided Wave-Based See-Through Display’, US 2010/0157400 A1 (2010), 15—H. Mukawa, ‘Optical Device and Virtual Image Display’, U.S. Pat. No. 7,453,612 (2008), 16—H. Mukawa, K. Akutsu, ‘Optical Device and Virtual Image Display Device’, U.S. Pat. No. 7,418,170 (2008), 17—Y-R. Song, Wearable Display System Adjusting Magnification of an Image′, US Pat. Application US2004/0004767 A1 (2004)]. The two holograms are coupled through a common substrate due to TIR. The chromatic dispersion of the first hologram is corrected by the dispersion of the second one, and the design is insensitive to light source wavelength drift/shift. While providing a means to mostly remove the color dispersion/shift from the imagery for the case when two identical holographic gratings are placed mirror-symmetrically on the waveguide, the developed systems require incorporation of additional optics that adds up to weight, volume, and cost. A desired reduction of the number of optical elements for such see-through systems was not clarified, except for [13—Ya. Amitai, A. Friesem, I. Shariv, ‘Planar Holographic Optical Device for Beam Expansion and Display’, U.S. Pat. No. 6,169,613 B1 (2001)], where a compact sight was proposed that incorporates a holographic lens coupled to a holographic grating through the total internal reflection (TIR) in the substrate. While indeed showing a good image quality for up to 4.0 mm diameter Eye Box size, the demonstrated Eye Box size is not large enough to provide an adequate see-through sight.
In the last ten years, software-based ray-trace techniques of designing holographic elements substantially advanced compared to traditional multi-step recursive analytical techniques described in, e.g. [13—Ya. Amitai, A. Friesem, I. Shariv, ‘Planar Holographic Optical Device for Beam Expansion and Display’, U.S. Pat. No. 6,169,613 B1 (2001)]. Using commercial off-the-shelf software such as, e.g., Zemax® or Code V®, it is quite possible to implement, in a robust and re-producible way, rather sophisticated holographic designs, thus revealing essentially new features in multi-component holographic systems. An additional advantage of such an approach compared to a traditional analytical recursive one is that it is rather straightforward to transfer a software-made design on an optical table and, actually, implement recording in practice.
What is desired is a method to design and fabricate substrate-guided holographic optical elements for a compact see-through weapon sight, which is reliable, reproducible, capable of providing large enough aperture sizes (e.g., up to 1.5 inch diameter), and extendable for low-cost mass production. In such a sight, in a compact form-factor, there is a thin transparent waveguide with holographic elements enabling in-coupling and out-coupling of light to/from the waveguide in such a way as to create a sufficient field-of-view, aberration-free virtual image of the reticle superimposed on the scene view, with a capability to provide a long (up to 100-200 mm) eye relief, and to provide a highly-transparent (˜90%) view of the outside world, with parallax reduced to unnoticeable, for a shooter level. At the same time, a monocular sight provides a see-through imagery without introduction of any noticeable color shifts/changes and laser speckle in the field-of-view and should be safe for a shooter's eye, and therefore avoid using laser illumination. The compact optical sight described above is the concept described in [13—Ya. Amitai, A. Friesem, I. Shariv, ‘Planar Holographic Optical Device for Beam Expansion and Display’, U.S. Pat. No. 6,169,613 B1 (2001)]. As was shown in [14—F. Dimov et al., ‘Holographic Substrate-Guided Wave-Based See-Through Display’, US 2010/0157400 A1 (2010)], such a system can be made of substantial aperture sizes (e.g., ˜1.5 inch diameter). It requires a broadband illumination source, provides a see-through imagery without color shift.